How do magnets alleviate pain?

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Does anyone know anything about magnets and how they help heal? I have arthritis and have some magnets that I put on my feet when I have severe pain. It does help quite a bit. But then someone told me I was using them incorrectly, that you are supposed to use the opposite poles. How do magnets work? Is there a correct way to use them? Does it need to be at or near the area in pain, or can they be placed anywhere?